This invention relates to a process for the recovery of halides from streams containing light hydrocarbons.
Numerous hydrocarbon conversion processes are widely used to alter the structure or properties of hydrocarbon streams. For example, isomerization processes rearrange the molecular structure from straight chain paraffinic hydrocarbons to more highly branched hydrocarbons that generally have a higher octane rating or increased utility as substrates for other conversion processes. Alkylation processes alkylate hydrocarbon alkylation substrates, such as aromatics or paraffins, with hydrocarbon alkylating agents, such as olefins, to produce motor fuels and useful industrial chemicals such as ethylbenzene, cumene, and linear alkyl benzenes that are used to make detergents. Additional processes include dehydrogenation, transalkylation, reforming, and others. Operating conditions and methods for carrying out these processes are well known by those skilled in the art.
Many of these processes share the common feature of using a catalyst in the presence of one or more materials that enhance the effectiveness of the catalyst in the reaction zone. These performance enhancing materials can operate in many ways, such as increasing or attenuating catalyst activity, neutralizing catalyst poisons, or solubilizing catalyst or feed contaminants. Such performance enhancement materials may be chemically or physically sorbed on the catalyst or dispersed in the hydrocarbon stream.
Where the hydrocarbon product stream leaving a hydrocarbon conversion zone contains the performance enhancing material or beneficent material, methods are sought for preventing contamination of the hydrocarbon product with the beneficent material and the loss of this beneficent material to the product stream. For example, many isomerization processes employ a highly effective platinum on chlorided alumina catalyst system in the reaction zone. The chlorided catalyst requires a continual addition of chloride to replace chloride lost from the surface of the catalyst in to the product stream. Hydrogen chloride and/or volatile organic chlorides escape from the process via a stabilizer overhead stream and, apart form the loss of chloride, pose environmental concern. In addition to the loss of chlorides and environmental concerns, chloride loss hinders the operation of chloride promoted isomerization zones in other ways. For example, the recycle of hydrogen or hydrocarbons through a zeolitic adsorption bed or a zeolitic conversion zone is not practical when a chloride type catalyst is used in the isomerization reaction zone, unless hydrogen chloride is removed from the recycle stream. Hydrogen chloride that is produced by the addition of chloride to the isomerization zone or that is released from the isomerization catalyst results in significant amounts of hydrogen chloride leaving in the effluent from the isomerization zone. Contact of this hydrogen chloride with a zeolite in, say, an adsorption bed or in a catalytic conversion zone, particularly in the presence of moisture and high temperature, will decompose the matrix structure of many zeolites, thereby destroying any adsorptive or catalytic function. Therefore, absent chloride neutralization methods, chlorided catalyst systems generally have insufficient compatibility with many zeolitic adsorbents or catalysts to permit simultaneous use.
Alkylation of hydrocarbons presents another case where contamination by a performance enhancement material can pose concern. Alkylation processes can use a solid chlorided alumina catalyst in the alkylation reactor, with the chloride acting as a performance enhancement material for the catalyst. In the course of alkylation, some chloride is lost from the catalyst into the product stream. In addition, the current commercial versions of these solid alkylation catalysts tend to experience fairly rapidly deactivation, which necessitates frequent regeneration, which in turn usually leads to the loss of more chloride from the catalyst into one or more regeneration effluent streams. Unless some or most of the lost chloride in the product stream and the regeneration effluent stream(s) is recovered and returned to the catalyst, the costs of neutralizing chlorides and of supplying fresh or make-up chloride to the catalyst can render alkylation processes that use solid chlorided catalysts less economical. Thus, methods have been sought for recovering and recycling materials, such as hydrogen chloride, which act to enhance or benefit the performance of catalysts in conversion zones when such materials are carried from the conversion zone by a hydrocarbon-containing effluent stream.
Since the 1960s, the chemical and petrochemical industries are increasingly using membranes in a broad variety of separation and recovery applications.
A membrane is a thin barrier having two sides and separating two fluids. In a membrane process, a feed stream passes to one side of the membrane, which is commonly called the feed side or nonpermeate side of the membrane. The feed stream contains a permeable component and a nonpermeable component, which is used herein to refer to a component that has a permeance that is less than that of the permeable component. The permeable component selectively passes through the membrane and is recovered on the reverse side, or permeate side, of the membrane in a stream which is called the permeate. The portion of the feed stream that does not selectively pass through the membrane, including the nonpermeable component, is recovered from the nonpermeate side of the membrane in a stream which is called the nonpermeate and which is also commonly referred to as the concentrate, the retentate, or the residue. General information on membrane separation processes can be found in Perry""s Chemical Engineers"" Handbook, Sixth Edition, edited by R. H. Perry and D. W. Green, published by McGraw-Hill Book Company, New York, in 1984.
In order for the permeable component to permeate from the nonpermeate side to the permeate side of the membrane at a particular location of a membrane, the local partial pressure of the permeable component at that particular location at or near the surface of the membrane must be greater on the nonpermeate side of the membrane than on the permeate side of the membrane. Local partial pressure of the permeable component means the product of the mole fraction of the permeable component and the total pressure, both determined locally at a given particular point at or near the surface of the membrane. In practice, the local mole fraction and local total pressure cannot be measured precisely at the surface of the membrane. Rather, the local mole fraction and total pressure are determined at least in part by the bulk flow rates, compositions, and pressures of the stream that are contacted with the membrane, and by controlling these streams, suitable local partial pressures of the permeable component can be controlled on both sides of the membrane.
The separation of gas streams using dense membranes that are nonporous and yet permeable is well known. Dense membranes consist of a dense film through which a pressure and concentration gradient will force the diffusion of certain components. The relative rates of transport of various components through the dense film does not necessarily depend on the size of the components, as much as it depends on the diffusivity and solubility of the components. See the article entitled xe2x80x9cMembrane Technology,xe2x80x9d written by Richard W. Baker, appearing at pp. 135-193 in Vol. 16 of the Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, published by John Wiley, in New York, in 1995.
Two of the chief criteria for a dense membrane separation process are permeance and selectivity. At this point, it is useful for the sake of clarity to define these terms:
i. Permeance
In simple permeation, the permeation flux of a species i across a membrane may be expressed by the formula,
Ri=Pi*(yH*Hxe2x88x92yL*L)
where Ri equals the rate of permeation in units of standard volume of species i per unit of time per unit of membrane cross-sectional area, Pi equals the permeance, yH equals the mole fraction of species i on the high-pressure or nonpermeate side of the membrane, H equals the pressure on the high-pressure side of the membrane, yL equals the mole fraction of species i on the low-pressure or permeate side of the membrane, and L equals the pressure on the low-pressure side of the membrane. Thus, permeance has units of standard volume of species i per unit of time per unit of membrane cross-sectional area per unit of pressure.
ii. Selectivity
Selectivity is defined as the ratio of the permeance of species i relative to the permeance of another species j and can be expressed by the equation,
Sij=Pi/Pj
In this equation, the convention is to put the numerically larger permeance in the numerator and the numerically smaller permeance in the denominator, so that the higher the selectivity of a membrane, the more desirable is the use of that membrane in a process for separating species i and j.
Examples of dense membranes include sulfonated fluorinated polysulfone membranes that comprise polymers, including copolymers, which have the polymer repeat unit of the following general type of structure in the polymer or copolymer, which is hereinafter referred to as formula (I): 
In the above formula (I), S is the sulfonic acid group (SO2OH) or its salified form, and n represents the average number of polymer repeat units in the polymer or copolymer. Hackh""s Chemical Dictionary, Third Edition, edited by Julius Grant, published by The Blakiston Company, Inc., New York, in 1953, defines the term xe2x80x9csalifyxe2x80x9d as xe2x80x9cto form a salt.xe2x80x9d The term xe2x80x9csalified form of the sulfonic acid groupxe2x80x9d as used herein means a form of the sulfonic acid group wherein the hydrogen atom of the hydroxyl group of the sulfonic acid group is replaced with a cation or cationic group. The salified form typically contains an ammonium group, an alkali metal atom, an alkaline earth metal atom, a transition metal atom, or an organic cation group. The polymer or copolymer has a molecular weight of generally above about 10,000 and preferably from about 25,000 to about 80,000. The polymer or copolymer has a degree of substitution (DS) of S groups of from about 0.2 to about 4. These sulfonated fluorinated polysulfone membranes are disclosed in U.S. Pat. No. 4,971,695 (Kawakami et al.), which teaches their use in separating gas mixtures such as air, as well as mixtures comprising hydrogen/nitrogen, hydrogen/methane, oxygen/nitrogen, ammonia/nitrogen, carbon dioxide/oxygen, carbon dioxide/methane, and hydrogen sulfide/methane. It is also known that these sulfonated fluorinated polysulfone membranes are used to separate gas mixtures of water/air and water/methane, with water permeating through the membrane.
Although sulfonated fluorinated polysulfone membranes have been used for separating streams, these membranes have not been used for separating and recovering hydrogen chloride from streams. Therefore, a method is sought that uses membranes for recovering and recycling hydrogen chloride from hydrocarbon-containing streams.
It has been discovered that sulfonated fluorinated polysulfone membranes have surprising and unexpected selectivity for separating compounds containing a halide, i.e., a Group 7A (IUPAC 17) element, such as hydrogen chloride, from streams that contain C1-C7 hydrocarbons. While it is already well known that sulfonated fluorinated polysulfone membranes are highly selective in permeating hydrogen from a mixture of hydrogen and methane, it has now been discovered hat sulfonated fluorinated polysulfone membranes are nearly as selective in permeating hydrogen chloride from a mixture with methane as they are in permeating hydrogen from a mixture with methane. In addition, because the permeance of sulfonated fluorinated polysulfone membranes toward hydrocarbons decreases as the number of carbon atoms in the hydrocarbon increases, the sulfonated fluorinated polysulfone membranes are even more selective in permeating hydrogen chloride from a mixture with hydrocarbons that are heavier than methane. This invention takes advantage of these unexpected permeation characteristics of sulfonated fluorinated polysulfone membranes and, accordingly, in one of its embodiments comprises a process for separating hydrogen chloride from streams that contain C1-C7 hydrocarbons. The sulfonated fluorinated polysulfone membrane are polymers, including copolymers, having the polymer repeat unit of the previously mentioned formula (I).
The embodiment of this invention for separating hydrogen chloride from streams that contain C1-C7 hydrocarbons has wide applicability in the field of hydrocarbon processing for removing hydrogen chloride from light off gas or fuel gas streams in oil refineries. Such gas streams are generated in hydrocarbon conversion processes which use chloride-promoted catalysts. Particular examples of this embodiment comprise improved processes for the conversion of hydrocarbons using chloride-promoted catalysts in which hydrogen chloride is recovered from effluent streams and recycled to a catalytic hydrocarbon conversion zone by the use of sulfonated fluorinated polysulfone membranes. Specific hydrocarbon conversion processes to which this invention is applicable include processes for the isomerization, alkylation, reforming, and dehydrogenation of hydrocarbons. Compared to the prior art process of caustic scrubbing and adsorption for removing hydrogen chloride from such streams, this invention effectively eliminates or substantially decreases the significant costs of purchasing fresh caustic or fresh adsorbent and of disposing of spent caustic or spent adsorbent. This invention also dramatically reduces the make-up rate of chloride precursors, such as hydrogen chloride, carbon tetrachloride, and perchloroethylene to hydrocarbon conversion processes.
This invention is generally applicable to isomerization processes that use a performance enhancing material. Although this invention is useful in those isomerization processes which recycle hydrogen-rich gas from the reactor effluent separator to the reactor, this invention is particularly applicable to those isomerization processes which eliminate recycling from the reactor effluent separator to the reactor. Processes without such recycling use relatively low quantities hydrogen-rich gas on a once-through basis, as described in U.S. Pat. Nos. 4,929,794 (Schmidt et al.) and 5,026,950 (Schmidt et al.). The advantage of this invention, when used in the context of a once-through process, arises from the fact that hydrogen permeates readily through the sulfonated fluorinated polysulfone membrane, even more readily than hydrogen chloride. Thus, in the course of recovering hydrogen chloride from the stabilizer off gas stream of a once-through isomerization process, this invention also recovers and recycles nearly all of the hydrogen in the stabilizer off gas as well. Although the amount of hydrogen that is present in the stabilizer off gas of a once-through isomerization process is, of course, relatively small because these processes require comparatively little hydrogen-rich gas in any event, whatever hydrogen is recovered and recycled in the course of recovering and recycling hydrogen chloride further decreases the quantity of make-up hydrogen-rich gas that these once-through processes require.
This invention has other novel and particularly advantageous aspects in its application to hydrocarbon conversion processes. For each hydrocarbon conversion process to which this invention is applied, the benefits of this invention can be further enhanced by the use of a sweep stream, or purge, stream for the permeate side of the membrane. Although it is well known that the driving force for permeation of a component through a membrane can be increased by sweeping or purging the permeate side of the membrane with a gas containing a low mole fraction of that component, each particular hydrocarbon conversion process offers a wide range of streams for possible use as the sweep stream. In other words, many streams in a hydrocarbon conversion process meet the minimum criterion of a sweep stream, namely a low mole fraction of the permeating component. However, some streams are better than others for use as a sweep stream, and certain embodiments of this invention comprise a particularly advantageous choice of sweep stream. For example, in the case of an isomerization process, the most advantageous sweep stream is a portion of the stabilizer bottoms that has been vaporized, while in a motor fuel alkylation process, wherein an olefin such as propylene alkylates a paraffin such as isobutane in the presence of a stoichiometric excess of isobutane, the most advantageous sweep stream is a portion of the recycle isobutane that has been vaporized. These particular sweep streams are preferred because they maximize the driving force for permeation while minimizing the cost of utilities and without compromising in any way the benefits of this invention, namely retaining hydrogen chloride while minimizing hydrogen chloride contamination of gaseous products and effluent streams.
As has been suggested previously, it is believed that this invention has much wider applicability than the selective removal of merely hydrogen chloride or even of merely chloride-containing compounds. Because sulfonated fluorinated polysulfone membranes exhibit such high permeance for hydrogen chloride, it is believed that these membranes will likewise exhibit surprisingly high permeance for some other chlorine-containing hydrocarbons that have a non-zero resultant molecular moment. Thus, it is expected that sulfonated fluorinated polysulfone membranes can be used to separate a chlorine-containing hydrocarbon from a mixture of hydrocarbons having the same or more carbon atoms as the chlorine-containing hydrocarbon. Moreover, it is believed that this high permeance exhibited by sulfonated fluorinated polysulfone membranes for substituted hydrocarbons is not limited to chlorine-containing hydrocarbons. Accordingly, it is expected that sulfonated fluorinated polysulfone membranes can be used to separate a substituted hydrocarbon containing any Group 7A element, not only chlorine, from a mixture of hydrocarbons having the same or more carbon atoms as the substituted hydrocarbon. Thus, in its broadest scope, this invention comprises the use of sulfonated fluorinated polysulfone membranes to recover a permeable component containing a Group 7A element from a stream containing the permeable component and a C1-C7 hydrocarbon.
In a broad embodiment, this invention is a process for recovering a permeable component containing a Group 7A element from a feed stream containing the permeable component and a nonpermeable component. A feed stream, which comprises a permeable component containing a Group 7A element and a nonpermeable component, passes to a membrane separation zone. The membrane separation zone comprises a resin membrane comprising a polymer or copolymer containing the polymer repeat unit represented by the previously mentioned formula (I), wherein S is the sulfonic acid group or its salified form and wherein n represents the average number of polymer repeat units in the polymer or copolymer and wherein the polymer or copolymer has a molecular weight above about 10,000 and a degree of substitution of S groups of from about 0.2 to about 4. A permeate stream comprising the permeable component is withdrawn from the membrane separation zone.
In another embodiment, this invention is a process for the isomerization of a normal paraffin. Normal paraffins are introduced to an isomerization reaction zone, where normal paraffins are isomerized to branched paraffins having the same number of carbon atoms as the normal paraffins. The isomerization occurs in the presence of a chloride-containing catalyst. A reaction effluent stream is recovered from the isomerization reaction zone. The reaction effluent stream comprises branched paraffins, hydrocarbons having from 1 to 7 carbon atoms, and hydrogen chloride. At least a portion of the reaction effluent stream passes to a product separation zone, which operates at conditions to separate the entering paraffins. A recycle stream comprising hydrocarbons having from 1 to 7 carbon atoms and hydrogen chloride is withdrawn from the product separation zone. At least a portion of the recycle stream passes to a membrane separation zone comprising a resin membrane comprising a polymer or copolymer containing the polymer repeat unit represented by the previously mentioned formula (I). A permeate stream is withdrawn from the membrane separation zone. The permeate stream comprises hydrogen chloride. Branched paraffins are recovered from the process.
In another embodiment, this invention is a process for the alkylation of an alkylation substrate with an alkylating agent. An alkylating agent and an alkylation substrate pass to an alkylation reaction zone. In the alkylation reaction zone, the alkylating agent alkylates the alkylation substrate in the presence of a chloride-containing catalyst to form alkylate. A reaction effluent stream is recovered from the alkylation reaction zone. The reaction effluent stream comprises alkylate, hydrocarbons having from 1 to 7 carbon atoms, and hydrogen chloride. At least a portion of the reaction effluent stream passes to a product separation zone, which operates at conditions to separate the entering hydrocarbons. A recycle stream comprising hydrocarbons having from 1 to 7 carbon atoms and hydrogen chloride is withdrawn from the product separation zone. At least a portion of the recycle stream passes to a membrane separation zone comprising a resin membrane comprising a polymer or copolymer containing the polymer repeat unit represented by the previously mentioned formula (I). A permeate comprising hydrogen chloride is withdrawn from the membrane separation zone. Alkylate is recovered from the process.
The isomerization of normal butane to isobutane is described in xe2x80x9cUOP Butamer Process,xe2x80x9d Chapter 9.2, by Nelson A. Cusher, in the book entitled Handbook of Petroleum Refining Processes, Second Edition, edited by Robert A. Meyers, and published by McGraw-Hill in New York in 1997. The isomerizations of normal pentane to isopentane and of normal hexane to branched hexanes are described in xe2x80x9cUOP Penex Process,xe2x80x9d Chapter 9.3, by Nelson A. Cusher in the Meyers book.
U.S. Pat. Nos. 4,929,794 (Schmidt et al.) and 5,026,950 (Schmidt et al.) describe isomerization processes that eliminate the recycling of a hydrogen-rich gas to the isomerization reactor.
U.S. Patent No. 5,489,732 (Zhang et al.) discloses a fluidized solid bed motor fuel alkylation process that uses a halide. The teachings of U.S. Pat. No. 5,489,732 are incorporated herein by reference.
The use of nonporous, dense membranes in gas separation is described in the article entitled xe2x80x9cMembrane Technology,xe2x80x9d written by Richard W. Baker, appearing at pp. 135-193 in Vol. 16 of the Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, published by John Wiley, in New York, in 1995. This article teaches that the relative rate at which a component permeates through a nonporous, dense membrane depends on the diffusivity and solubility of the component in the membrane material.
It is well known that the driving force for the permeation of a component through a membrane can be increased by purging the low-pressure, or permeate, side of the membrane with a gas containing a low mole fraction of that component. See, for example, pages 1-13 in Inorganic Membranes for Separation and Reaction, by H. P. Hsieh, Membrane Science and Technology Series 3, Elsevier Science B. V., New York, 1996; and pages 308-326 in Membrane Separation Processes, ed. by P. Meares, Elsevier Scientific, New York, 1976. See also the following three articles by C. Y. Pan and H. W. Habgood: Ind. Eng. Chem., Fundamen., Vol. 13, No. 4, 1974, pp. 323-331; Can. J. Chem. Engg., Vol. 56, April 1978, pp. 197-209; and Can. J. Chem. Engg., Vol. 56, April 1978, pp. 210-217.
The article entitled xe2x80x9cPollution Control: Field Tests Show Membrane Processing Attractive,xe2x80x9d by T. E. Cooley and W. L. Dethloff, CEP, October, 1985, pp. 45-50, describes a normal butane isomerization process that passes a hydrogen-containing feed gas stream to an undisclosed membrane in order to recover hydrogen as permeate and recycle hydrogen to the process. The article teaches that when the feed gas stream to the membrane contained hydrogen chloride, two undesirable effects occurred: 65% of the hydrogen chloride permeated through the membrane, and high levels of hydrogen chloride impaired the membrane performance. To avoid these effects, this article teaches caustic washing of the feed gas stream to the membrane so that the feed gas stream is free of hydrogen chloride.
U.S. Pat. No. 4,971,695 (Kawakami et al.) discloses a sulfonated fluorinated polysulfone membranes and their use in separating gas streams. The teachings of U.S. Pat. No. 4,971,695 are incorporated herein by reference.